1. Field of the Invention
Aspects of the present invention relate to a cylindrical lithium ion secondary battery, and more particularly, to a cylindrical lithium ion secondary battery having an improved stability with respect to external pressure.
2. Description of the Related Art
In general, a cylindrical lithium ion secondary battery includes an electrode assembly that is wound substantially in the form of a cylinder, a cylindrical can that is connected to the electrode assembly, an electrolytic solution that is infused inside the can to allow lithium ions to move, and a cap assembly that is connected to one side of the can and prevents the electrolytic solution from leaking and prevents the electrode assembly from separating.
The cylindrical lithium ion secondary battery typically has a capacity of approximately 2000˜2400 mA, and thus is mainly used in electronic devices requiring high power such as notebook PCs, digital cameras, or camcorders. For example, a required number of cylindrical lithium ion secondary batteries may be connected in series or parallel, may include a protection circuit, and may be assembled in a specific shape in a hard pack to be used as a power source connected to an electronic device.
The cylindrical lithium ion secondary battery is manufactured by the following method.
First, a negative electrode plate that includes an active material and a positive electrode plate that includes a separator and the active material are laminated together. One end thereof is then connected to a pole-shaped wind-axis, and is then wound in a substantial cylindrical shape, thereby forming an electrode assembly. Thereafter, the electrode assembly is inserted into a cylindrical can, an electrolytic solution is then infused therein, and a cap assembly is welded at an upper portion of the cylindrical can, thereby forming a lithium ion secondary battery having a substantial cylindrical shape.
Recently, a core member 100 having a substantial pole shape as shown in FIG. 1 has been inserted in the center of the electrode assembly, so that the electrode assembly is not deformed when the cylindrical lithium ion secondary battery is charged or discharged. The core member 100 is generally formed by winding a material that initially has a form of a plate, so that a tube shape having a circular cross-section is formed. A portion thereof is cut along a longitudinal direction.
However, in the cylindrical lithium ion secondary battery above, the core member 100 may move due to an external impact such as a drop. The movement of the core member 100 may affect the cap assembly located at an upper portion of the core member 100. In particular, when the core member 100 collides with a safety belt of the cap assembly, the safety belt may be inverted or damaged. If the safety belt is inverted or damaged, it may severely affect a safety of a lithium secondary battery.
Further, when the core member 100 rotates, a lower end of the electrode assembly may come in contact with the core member 100, and thus a short may occur at the lower end of the electrode assembly.
Furthermore, when the core member 100 is deformed or damaged, the separator of the electrode assembly may be damaged, and the damage to the separator may lead to a short between the positive electrode plate and the negative electrode plate, which may produce even greater damage or an explosion of the cylindrical lithium ion secondary battery. In particular, when external pressure is applied, the core member 100 may be deformed in a specific direction at a point where the external pressure is concentrated. The electrode assembly receives the pressure in the specific direction, thereby producing a short between the electrode plates.